The present invention relates to a humidity meter or a hygrometer which provides DC potential proportional to the measured relative humidity, in particular, relates to such a meter which has a linear relationship between the relative humidity and the output DC potential.
The present humidity meter is applicable to various kinds of electronic apparatuses, including a copying machine, a printer et al, and/or other field of application for visually indicating humidity.
Conventionally, a humidity sensor which provides electrical output relating to humidity has been known. One of them is an impedance change type humidity sensor, including a ceramics type humidity sensor and a polymer type humidity sensor. It is supposed that a water molecule couples with porous ceramics or porous polymer, which is ionized and provides electrical conductivity.
Although the impedance of a conventional humidity sensor relates to relative humidity, the relation between the impedance and the humidity is exponential. When the humidity is low, the impedance is very high, and when the humidity is high, the impedance is very low. A prior electronic circuit is not sufficient for following the wide range of change of the impedance of the humidity sensor.
FIG. 15 is a block diagram of a prior humidity meter. In the figure, the numeral 1 is a humidity-frequency converter, 2 is a differentiation circuit, 3 is a waveform shaping circuit, and 4 is an integrator.
The humidity-frequency converter (H-F converter) 1 provides the output pulses, the frequency of which relates to the humidity. The output pulse of the H-F converter 1 is applied to the differentiation circuit 2. The differentiated pulse is applied to the waveform shaping circuit 4, which takes the signal having the level higher than the predetermined threshold level, and waveform-shapes, and provides the rectangular output pulse. The output pulse of the waveform shaping circuit 3 is applied to the integrator 4, which integrates the input pulses and provides DC (direct current) output level, which relates to the frequency of the output of the converter 1 and the pulse width, or the humidity.
It should be noted in FIG. 15 that the differentiation circuit 2, the waveform shaping circuit 3 and the integrator 4 compose a frequency-voltage converter for converting the frequency to the voltage level by counting the number of pulses.
FIG. 16 is a practical circuit diagram of the prior humidity meter as described in FIG. 15. The same numerals in FIG. 16 show the same members as those in FIG. 16. The symbols G.sub.1 through G.sub.3 are a gate, R.sub.1 through R.sub.5 are a resistor, C.sub.1 through C.sub.4 are a capacitor, and HS is a humidity sensor which changes the impedance depending upon humidity.
The humidity-frequency converter 1 has a C-MOS gate IC (integrated circuit) with a gate G.sub.2 (buffer gate) and a gate G.sub.3 (inverter), the capacitor C.sub.1 and the resistors R.sub.1 and R.sub.2. Those elements compose an astable multi-vibrator type oscillator.
The series circuit of the humidity sensor HS, the resistor R.sub.3 and the capacitor C.sub.2 is coupled parallel with the resistor R.sub.2 of said oscillator for controlling the oscillation frequency depending upon the humidity. The resistor R.sub.1 functions to protect the gate G.sub.2. The resistor R.sub.2 and the capacitor C.sub.1 determine the oscillation frequency The capacitor C.sub.2 functions to prevent DC component applied to the humidity sensor HS. The resistor R.sub.3 functions to improve the characteristics of the humidity sensor HS in a high humidity region.
The oscillation frequency of the converter 1 is determined by the capacitor C.sub.1, the resistor R.sub.2 and the humidity sensor HS. When the humidity around the sensor HS changes, the impedance of the humidity sensor HS follows that change, and then, the oscillation frequency follows the change of the humidity. In other words, the pulse frequency of the oscillator 1 follows the humidity. The output of the oscillator 1 is differentiated by the differentiation circuit 2, which has the capacitor C.sub.3 and the resistor R.sub.4. The differentiated pulse is waveshaped by the waveform shaping circuit 3 which has the gate G.sub.1. The waveform shaped signal is integrated by the integrator 4 which has the resistor R.sub.5 and the capacitor C.sub.4. The integrated signal is the output which indicates the humidity.
However, a prior humidity meter has the following disadvantages.
1) The change of impedance for humidity is exponential. When humidity is low, impedance is very high, and when humidity is high, impedance decreases substantially. As the impedance changes in very wide range, the frequency change is also very large. For instance, the impedance changes from 10.sup.4 ohms to 10.sup.7 ohms. The circuit cannot follow that wide range of frequency, and cannot provide the linear output of humidity.
2) If the time constant of the differential circuit is designed for low humidity (low frequency), the circuit would saturate in high humidity, and the linearity of the circuit would be deteriorated. If the differentiation circuit is designed for high humidity, the output level for low humidity would be too low, and the linearity of the circuit would also be deteriorated.
Further, even if the differentiation circuit is designed for middle humidity, the circuit would not follow low humidity and high humidity, and the problem is not solved.
Therefore, the prior circuit can not follow the wide range of change of impedance of a humidity sensor.